The effects of engine speed and load on nitric oxide emission were investigated with a single-cylinder engine operated at conditions representative of those found in the 7-mode, 7-cycle Federal Emissions Test Cycle. The fuel-air mixture was constant at 83% of stoichiometric and the spark settings were optimized.Nitric oxide emission increased as engine speed and load were increased. The effects were interdependent, in that, the effect of speed was greatest at low load and the effect of load was greatest at low speed. In addition, both speed and load effects were larger when valve overlap was higher.Engine speed and load affected exhaust nitric oxide concentration primarily via influences on charge dilution. This finding explains the interdependency of the speed and load effects, and also why they depended on valve overlap.